We investigate electron attachment to large ammonia clusters doped with a single benzene (Bz) molecule (NH₃)_N·Bz, ≈ 320. Negatively charged clusters are probed by mass spectrometry, and the energy-dependent ion yields are derived from mass spectra measured at different electron energies. The ion efficiency curves of pure ammonia clusters exhibit two maxima. At around 6 eV, (NH₃)_n−1NH₂⁻ ions are produced via dissociative electron attachment (DEA) to NH₃ molecules. (NH₃)_n⁻ ions produced at this energy are formed by DEA followed by fragment caging. At low energies around 1.3 eV, only (NH₃)_n⁻ ions are formed for cluster sizes n ≥ 35 that correspond to solvated electrons in ammonia clusters. The doped (NH₃)_n·Bz⁻ cluster ions exhibit essentially the same energy dependence. The (NH₃)_n·Bz⁻ ions are metastable and evaporate NH₃ molecule(s), while pure (NH₃)_n⁻ ions are stable. The lifetime for NH₃ molecule evaporation from the Bz-doped clusters was estimated as τ ≈ 18 μs. We interpret the metastability of the doped clusters by the charge localization on a Bz⁻ ion solvated in the ammonia, which is accompanied by an energy release leading to the evaporation of NH₃ molecule(s).